This invention relates to a fluid powered actuator assembly for valves which are moved to the open position by application of fluid pressure to an actuator piston and are moved to the closed position by the relaxation of compressed return springs of the actuator, and more particularly to an actuator assembly for valves for effecting the shearing of wire lines or similar obstructions disposed through the valve opening.
With wellhead assemblies associated with oil and gas wells, it is oftentimes necessary to suspend wire lines, power cables, or tubing through the wellhead for supplying electrical power to downhole equipment, or for chemically treating production zones, or to perform other services. Typically, such lines or tubing are passed through a valve of the wellhead assembly or through a downhole safety valve controlled by a controls system located at the surface. In the event of an emergency, however, it may be urgently required that the well be immediately shut in. Consequently, the various wire lines, tubing, and other suspension members passing through the opening of the valve must be cut in order to quickly shut in the well.
Typically, such valves are actuated to their open position through the application of a control fluid pressure to the piston of a piston-type actuator. Also, the stroke of the actuator to open the valve compresses one or more return springs which are installed in the actuator to provide a mechanical means of returning the valve to its closed position without reliance on fluid control lines.
The force required to shear the wire lines, cables or tubing passing through the valve is frequently considerably greater than that required of an actuator piston return spring for merely closing the valve. In addition, during the return stroke, the thrust force of the piston return spring diminishes rapidly from its initial value so that the available force for severing wire lines, etc., is less than half the initial thrust. Accordingly, it is desireable to provide a mechanism which will provide an additional closing thrust to the actuator shaft as it approaches the fully closed position of the valve so as to avoid the necessity for developing very powerful, very large sized actuators capable of wire line shearing operations. One such apparatus is disclosed in U.S. Pat. No. 4,372,333 wherein a plurality of energy storing devices are radially disposed with respect to the actuator shaft and are energized to an energy storing condition by a cam surface on the actuator shaft during its initial movement in the valve opening direction. When the actuator shaft is returned in the reverse direction to close the valve by the piston return spring, the energy stored in the energy storage devices is released as additional axial thrust on the actuator shaft toward the valve closed position. However, it is difficult to achieve an additional thrust of sufficient magnitude to sever the larger sized workover lines and tubing without greatly enlarging the actuator itself. In addition, every actuator model incorporating such energy storage devices must be specifically designed for their accommodation, and the energy storage devices intended therefor must also be differently sized and varied in number.
Accordingly, it is an object of the invention to provide an actuator assembly having an auxiliary energy storage device which can be universally used with a wide variety of piston-type actuators of many different sizes used in valve operation to provide additional thrust to the actuator shaft during its return stroke as it approaches the closed position of the valve.
Another object is to provide an actuator assembly for operation of a gate valve having a flow port through the gate element thereof, which assembly comprises an auxiliary energy storage device of the coiled spring variety which is readily energized during the valve opening stroke of the actuator shaft and releasably maintained in the energized condition until its release during the return stroke of the actuator shaft to provide additional thrust to the actuator shaft sufficient to sever large sized wire lines or other suspension members passing through the flow port of the valve gate.
A further object is to provide a piston-type actuator for a ported gate valve with an auxiliary storage device to provide large additional thrust to the actuator shaft towards the end of its return stroke sufficient to sever large tubing, cables or other suspension members which are passed through the opening of the valve gate and wherein the auxiliary storage device is a small fraction of the weight of the actuator itself or a conventional piston-type actuator capable of supplying the same thrust force during its return stroke.